Despite U.S. intelligence officials’ repeated denials that the National Security Agency is collecting the content of domestic emails and phone calls, evidence is mounting that the agency’s vast surveillance network can and may already be preserving billions of those communications in powerful digital databases.

A McClatchy review of public records, statements by Obama administration officials and interviews with cyber and telecom security experts lends credence to assertions that the capability for such surveillance exists.

– FBI Director Robert Mueller told a Senate committee on March 30, 2011, that “technological improvements” now enable the bureau “to pull together past emails and future ones as they come in so that it does not require an individualized search.”

– The administration is building a facility in a valley south of Salt Lake City that will have the capacity to store massive amounts of records – a facility that former agency whistleblowers say has no logical purpose if it’s not going to be a vault holding years of phone and Internet data.

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– Security experts, including a former AT&T engineer, say that the NSA has tapped into fiber-optic cables carrying phone and Internet data in cities across the country.

Glenn Greenwald, a columnist for the British newspaper The Guardian who reported on the disclosures of NSA leaker Edward Snowden, said in a speech over the weekend that an upcoming story will describe a classified document that “talks about how a brand new technology enables the National Security Agency to redirect into its own repositories 1 billion cellphone calls every single day.”

“What we are really talking about here is a globalized system that prevents any form of electronic communication from taking place without it being stored and monitored by the National Security Agency,” Greenwald said in a webcast to the Socialism Conference in Chicago. “It means they’re storing every call and have the capability to listen to them at any time.”

Greenwald’s assertions and hints on the public record still offer only a cloudy picture, and the NSA has not disclosed that it is building a storehouse for trillions of digital recordings of phone calls and emails.

The NSA collected bulk data on Americans’ emails for years before that program was shut down in 2011 when intelligence officials could not provide evidence of its effectiveness, Democratic Sens. Ron Wyden of Oregon and Mark Udall of Colorado said in a statement Tuesday. The agency now collects data showing who sent and received emails under a narrower program focused on foreign terrorism suspects.

But the continuing revelations have poured kerosene onto a fiery, long-running post-9/11 debate over whether the NSA, in a scramble for tiny clues that might prevent terrorism, is breaching citizens’ Fourth Amendment rights to privacy.

Jonathan Turley, a George Washington University law professor, charged that what’s already known shows that the NSA has violated the Constitution and created “the greatest threat to privacy in the history of this country. . . . They have created a system that for the first time makes every citizen visible to the government at the simple pressing of a button.”

Michael McConnell, a Stanford University law professor and a former federal appeals court judge, agreed that the program is potentially “dangerous,” but he called the case that it is illegal “extremely weak.” He likened the NSA’s collection of general data, such as who emailed whom, to an airport search of all passengers.

The controversy has prompted members of Congress to put growing pressure on U.S. intelligence agencies to improve their transparency.

The NSA insists that it only eavesdrops on or collects phone content from Americans’ overseas calls, and then only if it has a documented foreign intelligence purpose, such as counterterrorism, and obtains approval from a secret court. The agency’s director, Army Gen. Keith Alexander, told a House of Representatives hearing last month that the programs and other intelligence have protected Americans from terrorist attacks “in a way that does not compromise the privacy and civil liberties of our citizens.”

National intelligence chief James Clapper, in a recent NBC television interview, called it “ludicrous” to suggest that the government wants to listen to the private phone calls of everyday Americans.

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During the House hearing, Gen. Alexander told Congress that the agency’s “partnerships” with telecom and Internet companies have helped disrupt more than 50 terrorist attacks over the last 12 years.

Deputy Attorney General Jim Cole testified that the NSA doesn’t know the identities of Americans whose phone numbers are collected and only gets access to call content when investigators have found enough evidence to target a foreign terrorism suspect. So-called “metadata” about billions of emails is treated similarly, officials said.

But even as the Justice Department pursued Snowden on espionage charges from Hong Kong to Moscow, The Guardian reported that the British spy agency GCHQ has tapped into more than 200 cables carrying the world’s phone calls and Internet traffic and is sharing the data with the NSA. By May 2012, it said, 250 NSA analysts had been assigned to help comb through the mountain of data.

Digitized records of calls over Google Voice, Skype and other “voice over Internet” systems, many of them to points overseas, would be among the records.

NSA spokeswoman Judith Emmel said that the “NSA does not ask its foreign partners to undertake any intelligence activity that the U.S. government would be legally prohibited from undertaking itself.”

NSA’s doubters point, in particular, to the agency’s push to build the massive data center in Bluffdale, Utah, whose storage capacity will be measured in the highest metric now used – “yodabytes,” named for Yoda, the “Star Wars” character.

Thomas Drake, a former NSA senior executive who challenged the data collection for several years, said the agency’s intent seems obvious.

“One hundred million phone records?” he asked in an interview. “Why would they want that each and every day? Of course they’re storing it.”

Drake, who fears that NSA chief Alexander is building “extraordinary power,” wound up as the fourth American to face charges under the Espionage Act after he blew the whistle on a wasteful NSA data-collection system. His prosecution, which Drake alleges was both retaliatory and an attempt to silence other potential leakers, eventually crumbled.

Lending credence to his worries, The Guardian’s latest report quoted a document in which Alexander purportedly remarked during a 2008 visit to an NSA intercept station in Britain: “Why can’t we collect all the signals all the time?”

A U.S. government official, who lacked authorization to speak for the record, said that Alexander’s comment was merely “a quip taken out of context.”

NSA officials say the leaks have undermined their attempt to cloak their operations to avoid tipping off terrorists to their sources and methods. But in a debate that touches the vast majority of Americans, the agency hasn’t won many points for candor.

The NSA kept secret for more than four years that it was conducting warrantless wiretaps on overseas callers as part of its hunt for al Qaida members and other terrorists.

Last week, the agency removed from its website a fact sheet describing how it interpreted its authority under a post-Sept. 11, 2001, USA PATRIOT Act provision through which it’s collected Internet records – after members of Congress pointed to an inaccurate statement. Wyden and Udall, who have led congressional scrutiny of the agency, wrote Alexander that the error, which is classified, was “significant, as it portrays protections for Americans’ privacy as being significantly stronger than they actually are.”

The two senators, members of the Senate intelligence committee, chided Alexander that such inaccuracies “can decrease public confidence in the NSA’s openness.”

In a separate letter to spy chief Clapper, Wyden and 25 other senators lamented having learned through an unauthorized disclosure that the Patriot Act had been “secretly reinterpreted to allow the government to collect the private records of large numbers of ordinary Americans.”

In addition, Clapper dashed off an apology letter to the Senate after acknowledging in a recent NBC television interview that he gave “the least untruthful” answer he could in denying in sworn testimony in March that the NSA had collected huge caches of data on Americans.

George Washington’s Turley called Clapper’s statement an admission of a lie. He chastised members of the Senate intelligence committee – led by Democratic Chairwoman Dianne Feinstein of California and ranking Republican Saxby Chambliss of Georgia – for failing to seek a Justice Department perjury investigation, especially since they knew the extent of the surveillance program.

“They didn’t give Roger Clemens a choice: Did you speak truthfully or just get as close as you could?” Turley said, referring to the former New York Yankees pitching star who had to win acquittal by a jury to fend off charges that he lied to Congress in denying steroid use.

Clapper appears to have played a game of semantics over the meaning of the word “collect.” James Bamford, author of “The Shadow Factory” and two other books about the NSA, said that under the agency’s formal definition, an intercept only occurs “when a communication becomes understandable to a human being.”

The Patriot Act gave intelligence and law enforcement authorities a huge tool for looking at phone and Internet data to help identify terror networks. It authorized them to obtain orders from the secret Foreign Intelligence Surveillance Court that directed telecom and Internet giants such as Google, Verizon, AT&T, Facebook, Microsoft and Apple to share email and phone data.

Effectively, the law enabled the intelligence agencies to keep up with technological leaps that have transformed the world of surveillance. Nearly all personal data now moves over fiber-optic cables, each of whose tiny filaments can use pulsating flashes of light to transmit up to 100 billion bits of data per second.

Major phone carriers currently do not store digital recordings of phone calls, except for those occurring under court-ordered wiretaps, said a former technology chief for a major telecom company, who spoke on condition of anonymity to avoid harming relationships.

That means that the NSA only could collect the content of calls by flagging targets going forward or by capturing data and storing some or all of it.

Jim Hayes, head of the California-based Fiber Optic Association who once wrote a classified paper on how to tap into a cable, said that it would be relatively easy for the agency to copy data from a cable.

One former U.S. security consultant, who spoke on condition of anonymity to protect his connections to government agencies, told McClatchy he has seen agency-installed switches across the country that draw data from the cables.

“Do I know they copied it? Yes,” said the consultant. “Do I know if they kept it? No.”

Spokesmen for phone and Internet providers Verizon and AT&T, both of which have been gagged by the national security statutes from discussing their court-ordered assistance to the NSA, declined to comment.